The present invention relates to a method of making a mask structure including two (2) or more masks for a color cathode ray tube (CRT). Particularly, the masks are positioned at a small distance from the CRT's phosphor screen and are separated from each other.
One such CRT having this type of mask structure is the mask-focusing color picture tube. In a mask-focusing color picture tube, different potentials are applied to the masks and an electrostatic lens is formed between the facing masks. The electron beam utility factor is significantly increased compared with a conventional shadow mask type color CRT. A mask-focusing color picture tube is described in Japanese Utility Model publication No. 38930/1972, and U.S. Pat. Nos. 2,971,117 and 3,398,309.
Another type of CRT which has the above described mask structure is described in Japanese Patent Publication No. 2698/1980. This color CRT has two masks. One mask acts as a color selection electrode and the other mask acts as an electron shield for preventing the other mask from being bombarded by electron beams and from being deformed by its rising temperature resulting from that bombardment.
In both types of color CRTs, the corresponding apertures of the masks must be aligned coaxially with the electron beams. However, it is difficult to make such a mask structure. Japanese Patent Publication No. 28188/1972 discloses a method of making such a mask structure. According to this method, on one surface of one shadow mask a glass insulating layer is formed. Then the glass insulating layer is etched from the shadow mask side to form apertures. After that, another shadow mask is attached on the glass insulating layer. The mask structure made by this method has the glass insulating layer between two shadow masks in an effective area. Therefore, it is difficult to press-form the mask structure into a curved shape. Further, the glass insulating layer is charged up by electron beam bombardment and electron beams passing through the apertures are affected by such charge. Thus this mask structure is not practical.
Recently a practical method for making such a mask structure for a CRT was proposed in Japanese Patent Application No. 141740/1981. According to this method, a plurality of apertured flat masks are stacked and fixed together with paraffin. Then they are simultaneously pressed into predetermined shape. However, after pressing the masks, they have to be separated from each other at a predetermined gap. At this time, thin shadow masks tend to be deformed and the corresponding apertures may be offset. Thus the beam utility factor can easily be reduced.